1. Technical Field of the Invention
The present invention relates to a light beam scanning optical system, and more particularly to a structure of a light beam scanning optical system incorporated in laser beam printers, facsimile and the like for deflectively collecting and scanning on a photoconductor a luminous flux which is provided with image information.
2. Brief Description of Related Art
Generally, a light beam scanning optical system utilized in laser beam printers and facsimiles basically comprises a semiconductor laser as a light source, a deflecting means such as a polygon mirror and a galvano mirror, and an f.theta. lens. The deflecting means is utilized for scanning a luminous flux emitted from the semiconductor laser at an equiangular velocity, however, a homogeneous image can not be obtained since there occurs a difference in the scanning speed over the range from a central portion to both edges in the main scanning direction at the light collecting surface. An f.theta. lens is therefore provided for rectifying the difference in the scanning speed.
The f.theta. lens is made by combining various concave lenses, convex lenses and the like, and lens planning is extremely complicated. Because of a number of surfaces to be ground, it is quite hard to improve accuracy in the manufacturing process and the cost is quite expensive. Moreover, there is a limitation in selecting a material which possesses good permeability.
Accordingly, in place of the f.theta. lens, the use of an elliptical mirror (Japanese Published Unexamined Patent Application No. 123040/1979), the use of parabolic mirror (Japanese Published Examined Patent Application No. 36127/1980) and the use of a concave reflector (Japanese Published Unexamined Patent Appliation No. 173212/1986) have heretofore been proposed. However, there are difficulties in manufacturing the elliptical mirror and parabolic mirror with further difficulty in obtaining high manufacturing accuracy.
When the concave reflector is used, it is necessary to prepare the same length of a concave reflector as that of the main scanning direction since the reflected light is being collected in the direction perpendicular to the surface of a photoconductor thereby causing the optical system itself to become impracticably large in size.